The immune system in man is extremely complex, comprised of dozens of functionally-unique cell subsets. Each of these subsets may have a special role in the protection from viruses or other challenges to the immune system;only now are we beginning to recognize the extent of this heterogeneity and correlate the presence and functional state of these cells with clinical status of subjects. We have focused on four main topics within this broad subject: The expression of cytotoxic effector molecules;the expression profile of chemokine receptors;the functional evaluation of cytolytic T cells;and the detailed cytokine profile of antigen-specific T cells.